Polarization diversity is widely used for base stations to solve the problem of multi-path fading at the receiving antennas. Traditionally this is implemented by using two offset perpendicular slots or two centered crossed slots at the ground plane to excite the patch in orthogonal directions, where these slots are rectangular in shape.
For an antenna with offset slots only a simple feeding network is required. However, such an antenna has poor input port isolation of around 18 dB which cannot satisfy current requirements for mobile communication applications. Indeed, isolation of more than 30 dB is one of the criteria for a dual-polarized antenna to provide a reasonable level of diversity gain.
For an antenna with crossed slots, this criterion can be satisfied by using a pair of balanced microstriplines to feed each slot. Nevertheless, this needs a complex feeding network that consists of an air-bridge. In order to obtain high isolation with a simple feeding network, two offset H-shaped (or modified H-shaped) slots are used to excite the patch for orthogonal polarizations. This antenna can acquire an isolation of more than 30 dB over a wide impedance bandwidth of around 20%. Most dual-polarized patch antenna designs are based on the slot/aperture feeding method. They do not involve the probe feeding method due to the strong coupling between the vertical metallic parts of the feeding probes.